This invention relates to media suitable for use as electrosensitive papers, more particularly, multi-layer electric or spark discharge papers.
Recording of electrical signals and processes for converting electrical signals directly into visible images have undergone much development since the early efforts of Western Union some 35 years ago. A number of these processes have been developed including dry electric discharge recording, electrolytic recording, and thermal recording. Of particular interest and utility in the area of communications is the electrical discharge recording method used in facsimile and other telecommunication devices.
In an electric discharge or spark discharge recording system an image is formed by an electrical discharge which passes to ground through a sheet comprising multiple layers of optically contrasting materials. The electrical discharge normally emanates from a moving needle-like electrode or stylus and causes a series of perforations in the surface layer to produce a desired optical pattern. These perforations result from a selective removal or destruction of portions of the surface layer which are "melted", "burned" or "exploded" away from the paper to expose portions of the underlying layer. Often the exposed underlying layer is black or blackish-brown and contrasts with the white or grayish-white surface layer.
Many multi-layered electrosensitive papers comprise top layers consisting of thin metal "sheets" and/or synthetic resin(s); and underlying (imaging) layers containing a synthetic resin and carbon black, pigments or other contrast agents.
These papers also normally comprise substrates, such as paper, which provide dimensional stability to their associated layers, and may optionally comprise additional layers such as base layers between the imaging layer and the substrate. Such base layers, if desired, may comprise thin sheets of electrically conductive materials.
These electrosensitive papers are made by creating the desired number and type of layers on the substrate using any of a variety of coating, lamination and/or deposition processes. For example, coating may be accomplished through solvent or dispersion coatings which may subsequently be treated, such as by heating, to effect completion of the desired layer. Please refer to U.S. Pat. Nos. 3,496,063; 3,875,023 (Re. 29,427); 3,898,672; 3,920,873; 3,901,769; 4,105,449; 4,071,666 and 4,067,780 for disclosures of various types of electrosensitive papers and methods for producing same.
It has long been known to coat various substrates, such as paper, with dispersions of polyolefins, such as low density polyethylene. For example, U.S. Pat. No. 3,432,339 discloses a process for coating various substrates, such as paper, with polymers such as polyolefins polymers alone or in combination with rubber latex. According to this patent, finely divided polyolefinic polymers having an average particle size from about 2 to 30 microns in an inert carrier liquid such as water are used. The resulting dispersion is applied to the surface of the substrate and then heated to evaporate the inert liquid. In accordance with one of the preferred embodiments, a minor amount of rubber latex is dispersed along with the finely divided polyolefinic polymer. Similarly, U.S. Pat. No. 3,676,189 discloses the coating of a polyolefinic surface, such as a polyolefin coated photographic paper, with aqueous silica sol, an acid stable and water insoluble film forming material (such as aqueous polyethylene emulsion), surface active agents, and an acid. This method is described as obviating the need for curing at elevated temperatures, such as 50.degree. C., as suggested in prior art organic film forming methods.
Coatings which may be satisfactory for other purposes, such as to improve water resistance, increase activity and alter other properties of a given substrate, are recognized as being inoperative or disadvantageous for use as layers in electrosensitive papers. A suitable composition for use in forming one or more layers of an electrosensitive paper must, in combination with the other layers of that paper, produce a product which exhibits uniform sensitivity, resistance to discoloration, suitable pattern resolution, uniform and predictable electrical conductivity/resistance, and resistance to the formation of offensive smoke and/or odors during recording. As mentioned above, the art has experimented with a wide variety of materials, including many resins, metals and fibers, and methods of processing same, for the purpose of achieving suitable end product properties. Various attempts to improve properties of given electrosensitive papers are described in U.S. Pat. No. 3,920,873 and U.S. Pat. No. 3,935,333. In U.S. Pat. No. 3,920,873 fragrance oils are added to the underlying layer of the recording medium to mask unpleasant odors formed during recording, while alternatively, loose or stray surface fibers, which are described as aiding in the formation of recording odor and odor persistance, are reduced through a calendaring process which is intended to decrease the number of stray surface fibers. Similarly, U.S. Pat. No. 3,935,333 discloses an electrosensitive recording medium wherein a substrate such as polyethylene is covered by an underlying opaque conductor layer and a semi-conductive surface layer. The semi-conductive surface layer, which preferably comprises a mixture of metallic oxides and polymeric binders, is intended to lower the electric voltage necessary to record an image, thereby reducing the amount of smoke and odor released during recording.
Electrosensitive recording papers comprising surface and intermediate layers containing substantial amounts of synthetic polymers have experienced considerable success in the electrosensitive paper market in spite of the offensive odors which are generated by such papers during recording. Applicants believe that a major cause of burn-off odor created during the electrical discharge recording process is due to the degradation of polymers which are contained in one or more layers of such paper. It is, of course, well known that various polymers produce offensive odors upon heating or degradation. For example, it is known that above 635.degree. F. polyethylene breaks down and begins to generate offensive odors. (See U.S. Pat. No. 3,076,720, column 4, lines 17-20.) Similarly, as discussed above, it is not presently known what other factors may be involved in producing offensive odors or in controlling those odors once they are formed during recording with electrosensitive papers. Thus, while other nonconductive binders, such as polyvinyl alcohol, polyvinyl acetate, polyvinyl chloride, ethyl cellulose, gelatin, butadiene-styrene copolymers, vinyl acetate-crotonic acid copolymers, polymethyl methacrylate, polyethyl methacrylate and the like have been suggested, electrosensitive papers incorporating such binders have yet to optimize the desired properties referred to above in combination with the desired low odor characteristics.
For disclosures relating to the use of various polyolefins, including high and low density polyethylenes, in the paper coating area please refer to U.S. Pat. Nos. 3,494,826; 3,436,325; 3,551,538, 3,758,661; 3,840,625. Of course the factors affecting the selection of a particular binder are complicated by the fact that interactions between particular binders and the other selected components of the electrosensitive paper system are likely to alter the performance of the resultant product. For example, carbon black, even in low amounts, is known to color various polyolefins, such as polyethylene, and might be expected to interfere with pattern resolution by contaminating a top layer containing a carbon black-sensitive binder. Similarly, other incompatabilities may be experienced in a given electrosensitive paper system which could be accurately predicted by examining the characteristics of the individual components thereof. (See Modern Plastics Encylopedia 1974-1975, page 82.)